Method of making permanent magnets



Dec. 28, 1965 J. J. JESMONT ETAL 3,226,266

METHOD OF MAKING PERMANENT MAGNETS Filed Feb. 7, 1962 BH Q0 B IN GAUSS(10 H. IN. OERSTEDS (I03) INVENTORS JOHN J. JESMONT SAMUELWEIMERSHEIMEIR WW W M 34756 ATTORNEYS United States Patent OfiiiceMETHOD OF MAKENG PERMANENT MAGNETS John I. .lesmont, Spotswood, andSamuel Weimersheimer,

Rockaway, N..l., assignors to US. Magnet & Alloy Corporation,Bloomfield, N1, a corporation of Delaware Filed Feb. 7, 1962, Ser. N171,641 4 Claims. (Cl. 1483) The present invention relates to permanentmagnets and more particularly to magnets of ferrous alloys containingaluminum, nickel and cobalt having improved magnetic properties, and toalloys therefor.

Alloys containing 6 to 11 percent by weight aluminum, 12 to percent byWeight nickel, 16 to 30 percent by weight cobalt, up to 7 percent byWeight copper and the remainder principally iron, generally known asAlnico V, have been widely utilized in the production of permanentmagnets. By chill casting of the alloy under controlled conditions, ithas been possible to produce substantially unidirectionally grainedcastings which develop high magnetic properties. One of the principalproblems in achieving optimum properties is the tendency for the castingto develop a multiphase microstructure, whereas the ideal structuremagnetically is one consisting entirely of the alpha phase.

Accordingly, the heat treatment of Alnico alloys of the above analysisunder closely controlled conditions has been necessary to avoidmulti-phase structure and to achieve the desired alpha phase. Attemperatures of about 2350 F. and about 1650 F. only the alpha phaseexists but prolonged heating at temperatures of about 1525 to 1600 F.produces a beta phase and at temperatures of about 1750 to 2300 F.precipitates the gamma phase. Various elements have been proposed in aneffort to minimize the gamma phase problem and improve the magneticproperties including zirconium and silicon.

It is an object of the present invention to provide a novel permanentmagnet having greatly enhanced mag netic properties and characterized bysubstantial freedom from multi-phase structure.

Another object is to provide an alloy and method for making permanentmagnets which permits the facile casting of the magnet bodies and whichmay be readily heattreated without extensive normalizing treatment athigh temperatures to produce magnets of high magnetic strength and freefrom multi-phase structure.

A further object is to provide an alloy which can be readily cast into abody having a unidirectionally oriented grain structure and heat-treatedto provide a magnet having greatly enhanced magnetic properties.

Other objects and advantages will be readily apparent from the followingdetailed specification and appended claims, and by reference to theattached drawing wherein:

FIG. 1 is a graph showing the demagnetization curves of directionallygrained magnets and random-grained magnets of the alloy of the presentinvention and that of directionally grained magnets of conventionalAlnico V alloy not incorporating the additive elements of the presentinvention; and

FIG. 2 is a representation of the grain structure of directionallygrained magnets produced from the alloy of the present invention.

It has now been found that the foregoing and related objects can beattained by incorporation of 0.15 to 0.50 percent by weight columbiumand 0.15 to 0.50 percent by weight titanium in an alloy containing 6 to11 percent by weight aluminum, 12 to 20 percent by weight nickel, 16 to30 percent by weight cobalt, up to 7 percent by weight copper and theremainder substantially entirely iron with the usual impurities, such assilicon and carbon. The

3,226,266 Patented Dec. 2%, 1965 columbium and titanium are preferablyemployed in the range of 0.20 to 0.35 percent by weight of each foroptimum effect.

The preferred alloys of this invention are those ferrous base alloyscontaining 8.0 to 8.8 percent by weight aluminum, 13.0 to 14.0 percentby weight nickel, 23.5 to 25.0 percent by weight cobalt, 1.50 to 3.50percent by weight copper, to which the columbium and titanium are addedin the range of 0.15 to 0.50 percent by weight each, and most desirably0.20 to 0.35 percent by weight.

In the alloys of the present invention, the coercive force andmechanical properties are significantly enhanced while the problemsattendant to producing a single-phase microstructure are greatlyreduced, thus enabling the facile production of superior permanentmagnets. By the present invention, directionally grained magnets havinga maximum energy product (BH of at least 6.0 million gaussoersteds, andgenerally at least 7.0 million gaussoersteds are readily produced. Usingpreheated chill molds with little transverse heat loss, magnets havebeen produced in commercial operation having a maximum energy product of7.5 to 8.5 million gaussoersteds.

The alloy of the present invention may also be cast to producerandom-grained magnets in conventional baked sand molds with resultantproperties of coercive force and maximum energy product equivalent tocommercial directionally grained Alnico V magnets heretofore available.By the present invention, randorn-grained magnets having a coerciveforce of at least 700 oersteds and a maximum energy product of at least5.0)(10 gaussoersteds are readily obtained.

FIG. 1 of the attached drawing is a graph showing the demagnetizationcurves of directionally grained magnets (A) and of random-grainedmagnets (B) cast in commercial operation from the alloy of the presentinvention as compared with the demagnetization curve of directionallygrained magnets commercially produced from alloys theretofore utilized(C). FIG. 2 is a representation of the grain structure of directionallygrained magnet bodies commercially cast from the alloy and evidences butsix columnar grains across the transverse section.

The following is a specific example of an alloy of the present inventionwhich has proven highly advantageous for producing superior magnets:

Remainder iron with minor impurities.

Indicative of the properties of typical magnets produced from the alloyof the present invention are the data set forth in Table One below:

TABLE ONE Magnetic properties of magnets Residual Coercive Maximum En-Magnet Flux Force, He ergy Precinct Density, Br, Oersteds BH max,Gaussgausses Oersteds (X Direetionalized 13, 370 744 6.9

For casting directionally grained magnet bodies in accordance with thepreferred aspect of the present invention, the alloy preferably issuperheated to a temperature of 2850 to 3450 F. and cast in molds whichhave been preheated to a temperature of 1200 to 2700 F. and th superheatextracted through a chill plate at one end to achieve a unidirectionallygrained structure.

The alloy of the present invention is particularly advantageous in thatit substantially ameliorates the problems attendant to avoiding theproduction of a multiphase micro-structure in the magnets so as toobtain optimum magnetic properties. More particularly, it has been foundthat the castings of the present invention do not require normalizingtreatment at elevated temperatures above about 2300" F. to eliminate anygamma phase precipitation which may have occurred during casting.Furthermore, the castings of the new alloy may be heattreated attemperatures and times which enable facile and highly effectiveproduction of the magnetized bodies due to eliminating major problems inthe development of a multi-phase structure during heat-treatment.

In particular, the magnet castings of the new alloy are magneticallystressed by soaking at a temperature of 1650 to 1700 F. for fifteen totwenty minutes and cooling in a magnetic field parallel to the principalgrain direction to a temperature of about 1200 F. in about ten tofifteen minutes. Subsequently, the castings are coercively aged or drawnat a temperature of 1100 to 1150 F. for three to ten hours and then at1025 to 1100" F. for ten to twenty-four hours, after which the castingswere cooled to room temperature. The conditions for the magnetic stresstreatment are quite critical, but the condi- 'tions for the coerciveaging are relatively broad and free from any significant problems.

The coercive aging precipitates intermeta'llic compounds in the matrixand enhances the magnetic properties which have been developed byfreeing of the domains during soaking and alignment thereof duringcooling in the magnetic field parallel to the direction of grain growth.

Indicative of the cfficacy of the present invention are the followingspecific examples:

EXAMPLE ONE An alloy containing 8.3 percent by weight aluminum, 13.3percent by weight nickel, 24.5 percent by weight balt, 2.9 percent byweight copper, 0.2 percent by weight titanium, 0.3 percent by weightcolumbium, and the remainder principally iron with carbon and siliconimpurities of 'less than 0.12 percent was superheated to a temperatureof about 3350 F. and cast in refractory molds preheated to a temperatureof about 2050 F. and using a chill plate to produce speaker plugs 0.759inch in diameter and 0.522 inch in length. Upon inspection of atransverse section, the castings were found to be unidirectionallygrained and to have only six columnar grains.

The castings were then normalized at about 1700 F. for one-half hour andcooled in a magnetic field parallel to the axis of grain orientation forabout fifteen minutes. The magnet bodies were then subjected to coerciveaging at a temperature .of about 1100 F. for about two hours and atabout 1025 F. for twenty hours.

The magnets had a residual flux density (13,) of 13,400 gausses, acoercive force (H of 810 oersteds and a maximum energy product (BH of8.1 gaussoersteds.

EXAMPLE TWO An alloy containing 8.5 percent by weight aluminum, 13.0percent by weight nickel, 23.8 percent by weight cobalt, 2.6 percent byweight copper, 0.3 percent by weight titanium, 0.2 percent by weightcolumbium, and the remainder principally iron was superheated to atemperature of about 3450 F. and cast in refractory molds preheated to atemperature of about 2000 .F. and using a chill plate to produce speakerplugs two inches in diameter and two inches in length. Upon inspectionof a transverse section, the castings were found to be unidirectionallygrained and to have only 40 columnar grains.

The castings were then heat-treated similarly to those in Example One.

The magnets had a residual flux density (B,.) of 13,200 gausses, acoercive force (H of 810 oersteds, and a maximum energy product (BH of7.60 10 gaussoersteds.

EXAMPLE THREE The alloy of Example One was cast into speaker plugs ofthe same size in conventional sand molds according to conventionalpractice. The resultant castings were then magnetically heat-treatedunder conditions substantially identical to those in Example One.

The resultant magnets had a residual flux density (B of 12,750 gausses,a coercive force (H of 690 oersteds and a maximum energy product (BH of5.45 10 gauss oersteds.

EXAMPLE FOUR An alloy containing 8.3 percent by weight aluminum, 13.6percent by weight nickel, 24.3 percent by weight cobalt, 2.9 percent byweight copper, 0.15 percent by weight titanium and 0.2 percent by weightcolumbium was cast in conventional sand mOlds to form speaker plugs0.998 inch in diameter and 0.64 inch in length.

The resultant castings were heat-treated substantially in accordancewith the conditions in Example One. The resultant magnets were found tohave a residual flux density (B,.) of 12,500 gausses, a coercive force(H.,) of 715 oersteds and a maximum energy product (BI-1 of 5.6 10gauss-oersteds.

As can be seen from the foregoing specific examples and detailedspecification, the alloy of the present invention produces magnetshaving magnetic properties superior to those of conventional Alnico Valloys and may be utilized to cast unidirectionally grained structureshaving greatly enhanced magnetic properties over prior commercialdirectionally grained magnets. Additionally, the new alloy does notrequire as close control over temperatures and times to produce thedesired alpha phase microstructure.

We claim:

1. The method of making single-phase unidirectionally columnar grainedmagnets comprising: introducing into one end of mold chambers opening atthe other end on a chill plate a ferrous alloy containing about 6.0 to11.0 percent by weight aluminum, 12.0 to 20.0 percent by weight nickel,16.0 to 30.0 percent by weight cobalt, up to 7.0 percent by weightcopper, 0.15 to 0.50 percent by weight titanium, 0.15 to 0.50 percent byweight columbium, and the remainder being substantially all iron withminor impurities; chilling said ferrous alloy substantially through saidchill plate to form magnet bodies of the desired configuration withunidirectionally columnar grains; magnetically stressing said magnetbodies by soaking at a temperature of about 1650 to 1700 F. for aboutfifteen to twenty minutes and cooling in a magnetic field parallel tothe principal grain direction to a temperature of about 1200 in a periodof about ten to fifteen minutes; coercive aging said magnet bodies at atemperature of about 1100 to 1150 F. for three to ten hours andthereafter at a temperature of about 1025 to 1100 F. for ten totwenty-four hours to precipitate intermetallic compounds in the matrix;and cooling to room temperature.

2. The method in accordance with claim 1 wherein said copper constitutesabout 1.5 to 3.5 percent by weight of said alloy.

3. The method in accordance with claim 1 wherein said ferrous-base alloycontains about 8.0 to 8.8 percent by weight aluminum, 13.0 to 14.0percent by weight nickel, 23.5 to 25.0 percent by weight cobalt, up to7.0 percent by weight copper, 0.15 to 0.50 percent by weight titanitun,0.15 to 0.50 percent by weight columbium, and the remainder beingsubstantially all iron with minor impurities.

4. The method in accordance with claim 1 wherein said ferrous-base alloycontains about 8.0 to 8.8 percent 5 6 by Weight aluminum, 13.0 to 14.0percent by Weight 2,499,861 3/1950 Hansen 148-402 X nickel, 23.5 to 25.0percent by weight cobalt, up to 7.0 2,499,862 3/1950 Hansen 75124percent by weight copper, 0.20 to 0.35 percent by weight 2,694,16611/1954 Hadfield 148-102 X titanium, 0.20 to 0.35 percent by Weightcolumbium, and 2,797,161 6/1957 Ireland et al. 75124 the remainder beingsubstantially all iron With minor 5 FOREIGN PATENTS 601,597 7/1960Canada.

impurities.

References Cited by the Examiner UNITED STATES PATENTS DAVID L. RECK,Primary EXIUIIIIZGI.

21619.26 6/1939 Jonas 148 1O2 10 MARCUS U. LYONS, ROGER L. CAMPBELL,2,323,944 7/1943 Snoek 148-102 Ewmmm-

1. THE METHOD OF MAKING SINGLE-PAHSE UNIDIRECTIONALLY COLUMNAR GRAINEDMAGNETS COMPRISING: INTRODUCING INTO ONE END OF MOLD CHAMBERS OPEININGAT THE OTHER END ON A CHILL PLATE A FERROUS ALLOY CONTAINING ABOUT 6.0TO 11.0 PERCENT BY WEIGHT ALUMINUM, 12.0 TO 20.0 PERCENT BY WEIGHTNICKEL, 16.0 TO 30.0 PERCENT BY WEIGHT COBALT, UP TO 7.0 PERCENT BYWEIGHT COPPER, 0.15 TO 0.50 PERCENT BY WEIGHT TITANIUM, 0.15 TO 0.50PERCENT BY WEIGHT COLUMBIUM, AND THE REMAINDER BEING SUBSTANTIALLY ALLIRON WITH MINOR IMPURITEIS; CHILLING SAID FERROUS ALLOY SUBSTANTIALLYTHROUGH SAID CHILL PLATE TO FORM MAGNET BODIES OF THE DESIREDCONFIGURATION WITH UNIDIRECTIONALLY COLUMNAR GRAINS; MAGNETICALLYSTRESSING SAID MAGNET BODIES BY SOAKING AT A TEMPERATURE OF ABOUT 1650TO 1700*F. FOR ABOUT FIFTEEN TO TWENTY MINUTES AND COOLING IN A MAGNETICFIELD PARALLEL TO THE PRINCIPAL GRAIN DIRECTION TO A TEMPERATURE OFABOUT 1200*F. IN A PERIOD OF ABOUT TEN TO FIFTEEN MINUTES; COERCIVEAGING AGING SAID MAGNET BODIES AT A TEMPERATURE OF ABOUT 1100 TO 1150*F.FOR THREE TO TEN HOURS AND THEREAFTER AT A TEMPERATURE OF ABOUT 1025 TO1100*F. FOR TEN TO TWENTY-FOUR HOURS TO PRECIPITATE INTERMETALLICCOMPOUNDS IN THE MATRIX; AND COOLING TO ROOM TEMPERATURE.